runtime: print instruction bytes when reporting a SIGILL

Print the bytes of the instruction that generated a SIGILL.
This should help us respond to bug reports without having to
go back-and-forth with the reporter to get the instruction involved.
Might also help with SIGILL problems that are difficult to reproduce.

Update #37513

Change-Id: I33059b1dbfc97bce16142a843f32a88a6547e280
Reviewed-on: https://go-review.googlesource.com/c/go/+/221431
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/signal_unix.go

@@ -607,6 +607,30 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 		print("signal arrived during cgo execution\n")
 		gp = _g_.m.lockedg.ptr()
 	}
+	if sig == _SIGILL {
+		// It would be nice to know how long the instruction is.
+		// Unfortunately, that's complicated to do in general (mostly for x86
+		// and s930x, but other archs have non-standard instruction lengths also).
+		// Opt to print 16 bytes, which covers most instructions.
+		const maxN = 16
+		n := uintptr(maxN)
+		// We have to be careful, though. If we're near the end of
+		// a page and the following page isn't mapped, we could
+		// segfault. So make sure we don't straddle a page (even though
+		// that could lead to printing an incomplete instruction).
+		// We're assuming here we can read at least the page containing the PC.
+		// I suppose it is possible that the page is mapped executable but not readable?
+		pc := c.sigpc()
+		if n > physPageSize-pc%physPageSize {
+			n = physPageSize - pc%physPageSize
+		}
+		print("instruction bytes:")
+		b := (*[maxN]byte)(unsafe.Pointer(pc))
+		for i := uintptr(0); i < n; i++ {
+			print(" ", hex(b[i]))
+		}
+		println()
+	}
 	print("\n")
 
 	level, _, docrash := gotraceback()

test/fixedbugs/issue37513.dir/main.go

@@ -0,0 +1,27 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"os/exec"
+)
+
+func main() {
+	if len(os.Args) > 1 {
+		// Generate a SIGILL.
+		sigill()
+		return
+	}
+	// Run ourselves with an extra argument. That process should SIGILL.
+	out, _ := exec.Command(os.Args[0], "foo").CombinedOutput()
+	want := "instruction bytes: 0xf 0xb 0xc3"
+	if !bytes.Contains(out, []byte(want)) {
+		fmt.Printf("got:\n%s\nwant:\n%s\n", string(out), want)
+	}
+}
+func sigill()

test/fixedbugs/issue37513.dir/sigill_amd64.s

@@ -0,0 +1,7 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+TEXT ·sigill(SB),0,$0-0
+	UD2	// generates a SIGILL
+	RET

test/fixedbugs/issue37513.go

@@ -0,0 +1,9 @@
+// buildrundir
+
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build linux,amd64 darwin,amd64 linux,386
+
+package ignored